1. Technical Field
The present invention relates generally to an improved data processing system and in particular to the transmission of data in a bus device. Still more particularly, the present invention relates to an apparatus and a computer implemented method for a PCI Express component that operates in multiple modes.
2. Description of the Related Art
In data processing systems such as computers, a bus system is used to allow various parts of a computer to communicate with each other. For example, a bus system may connect a peripheral device to a central processing unit. A familiar example of this type of bus system is a universal serial bus (USB) system which allows a device, such as a mouse, to communicate with a mouse controller, which in turn is controlled by the computer's central processing unit.
One type of a known bus system is a peripheral control interconnect bus system, or a PCI bus system. A PCI bus system may be used to allow a computer component, such as a video card, to communicate with the video card controller and, in turn, with the computer's central processing unit. A modern version of the PCI bus system, known as peripheral control interconnect express (PCI Express) has recently been developed. The PCI Express bus system is a more advanced version of the previous PCI bus system, in the sense that a PCI Express bus system may rout data more quickly than a PCI bus system.
In the PCI Express bus system, the target to which data is routed is a PCI Express component, such as a device controller. In the previous paragraph, the video card controller would be a PCI Express component. The PCI Express component is configured for operation by software, such as a device driver. In addition, the PCI Express bus system includes a root complex component that performs the processing and logic required to allow a processor to communicate with the PCI Express component. In the case of a video card, the root complex component is the bridge between an express bus that connects to the video card controller and the host central processing unit. Continuing the above example, the video card controller is configured for operation by a device driver running on the host central processing unit through the topology of the root complex of the PCI Express bus system.
In an exemplary hierarchy, one root complex exists in a PCI Express hierarchy. In this case, the root complex is the only hierarchy agent that can run the PCI Express configuration and input/output cycle types required to configure the other devices on the hierarchy to achieve a functional bus topography. In addition to hierarchy configuration, the root complex has other unique features in the topology regarding PCI Express message handling, bus/device number assignment, and PCI Express bus initialization and training behavior.
Thus, in many types of data processing systems, a root complex connects the processor to the PCI Express component directly via a PCI Express bus. However, in some types of data processing systems, such as where the processor is logically or physically separated from the controlled device, an intermediate interface is required. In this case, two PCI Express bus systems are used, one for each data processing system. A first root complex is used to connect the CPU to an intermediate interface that connects to the other data processing system. A second root complex is used to connect a controlled device to the intermediate interface.
A problem associated with the two different data processing system configurations described above is that at least two application specific integrated circuits must be developed for each controlled device, even though the controlled devices are the same in each configuration. The problem arises because a root complex PCI Express component is used when the CPU and the controlled device are physically or logically separate, whereas a standard endpoint PCI Express component is used for devices within the same server enclosure. The PCI Express specification standards have different protocols for processing data through each type of PCI Express component. For this reason, a different application specific integrated circuit would normally have to be developed for each of the two different data processing system configurations, even though the controlled devices are the same in each configuration. However, the application specific integrated circuits used for both modes of operation share common features, such as the ability to transfer data to and from the controlled device.